The invention relates generally to chemical sensing and more particularly to fiber grating-based chemical sensing devices, systems and methods.
Fiber optic sensing methods have advantages including flexibility, small size, negligible weight, passivity, and immunity to electromagnetic interference. These characteristics make fiber optic sensors ideally suited for monitoring dynamic chemical and physical processes that are associated with changes in environment. The cascading capability of fiber optic sensing enables a distributed sensing application for large-area infrastructure embodiments such as oil tanks, pipelines, and power grid utilities in which fiber optic chemical sensing devices are embedded in multiple locations, some of which may be otherwise inaccessible.
Various sensing devices have been used. For example, some chemical sensing embodiments depend on dye molecules that absorb a particular wavelength of light and re-emit the light a short time late at a longer wavelength. Other sensing devices are employed to detect chemical parameters such as pH-value or corrosion, toxic chemicals, organic and inorganic fouling, and metal ions in an industrial processing system. These chemical sensing devices typically include electrochemical sensors, surface plasma resonance sensors, and attenuated total reflection sensors. Most of these sensors are limited by the operational conditions in which the sensors are employed. For example, conventional optical and electrochemical sensing devices are often limited to relatively mild pH conditions and, as such, limited operational temperature ranges. More specifically, conventional devices are typically limited to a pH range from about 5 to about 11 and temperatures lower than about 100° C.
Several fiber optic-based chemical-sensing devices have been used with examples including chemically active tapered fiber tips, chemically active fiber bundle distal ends, and chemically active fiber cladding embodiments. These chemical sensing devices typically include fluorescent dye immobilized doping, chemical sensitive thin film coatings, or combinations thereof. Chemical sensing with these fiber devices is based either on the reflectance intensity or on the fluorescence intensity. However, it is difficult to use these fiber optic sensors as real-time monitoring sensing devices because the fluctuation of the light source and environmental sensitive fluorescent dye luminescence limit measurement accuracy.
It is difficult to identify different chemicals in an environment like a water exchanger, a pipeline, oil and gas storage tanks, or an industrial processing system. Further, for large scale systems, a relatively large number of discrete chemical sensors may be required to map characteristics such as the structural health condition, pH-value, metal ion concentration, or inorganic fouling and scaling. Discrete optical, electrochemical and fiber optic sensors may not be scalable to meet a desired spatial resolution that is beneficial for accurate mapping of system conditions.
Short-period fiber Bragg grating (FBG) sensing devices have been used for refractive index measurement, pH-value sensing, and chemical identification. These sensing devices are typically based on D-type fibers, cladding etched fibers, or side-polished fibers. Such sensors utilize a wavelength encoding within a core of the sensor to measure a parameter based upon a Bragg wavelength shift that is generated on illumination of the grating through an illumination source. Thus, environmental effects on the periodicity of the grating alter the wavelength of light reflected, thereby providing an indication of the environmental effect, such as, pH, salinity, or temperature, for example.
Conventional telecommunication type FBG sensing devices have very little sensitivity to chemicals because the evanescent wave field is confined inside the fiber core (at a distance of about one wavelength from the fiber core and cladding interface). In order to make such FBG sensing devices chemically active, the fiber is mechanically modified to have an asymmetrical shape such as a D-shape, or is polished on one side, or has cladding thinned by a hydrofluoric acid etching process for example. In these processes, the fiber gratings are carefully prepared by polishing, cleaving, chemical etching, or shape- or polarization-controlled fabrication processes to turn a conventional fiber into a chemical sensing device. Processes to modify either the fiber cladding thickness or the fiber shape are intended to expose the evanescent wave field of the fiber core mode to an external medium.
It is difficult to simultaneously detect multiple parameters, such as pH, salinity, corrosion, and temperature, through a single mechanically or chemically modified Bragg grating sensing device. Further, multiple spectral signals at different wavelengths may be required to separate the effect of multiple sensed parameters from one another. Such separation of sensed parameters is difficult and time-consuming. Additionally, it is particularly challenging to fabricate multiple sensors on a single fiber cable when chemical etching a fiber with an inscribed Bragg grating structure. In principle, these fiber optic sensors have potential distribution capability, but, as a practical matter, it is not an easy task to handle multiple delicate sensors. In addition, the weak mechanical strength due to the thinner fiber grating cladding thickness can create failure modes.
Fiber long-period grating (LPG) sensors have demonstrated high sensitivity to surrounding medium refractive index variations due to forward cladding modes and core mode coupling efficiency. In LPG-based chemical sensors, the light in the fiber core is shed into the fiber cladding region. The resultant transmission loss depends strongly upon surrounding medium refractive index. Despite the removal of the cladding etch requirement, it is difficult to distribute many LPGs in a common chemical sensing system due to cladding modes crossover. Typically, within a bandwidth of one hundred nanometers, for example, the number of the LPG sensors is limited because of the multiple transmission peaks from each LPG. More practical difficulty lies in the detection of the chemical while other environmental factors such as temperature are varying. The different temperature and chemical sensitivities of the transmission peaks make it difficult to distinguish the chemical signal from the environmental influence.
Although cladding modes numbers or crossover can be reduced for LPG sensors by chemical etching for a LPG-based sensing device, the same mechanical processing challenge occurs as that associated with mechanically modifying short-period FBGs.